Invalid Postbak or call back argument issue

I have a website abc.com/index.html. i have following code in it
<form id="login-form" method="post" action="http://def.com/default.aspx"> <fieldset> <input name="hdnHidden" id="hdnHidden" value="-1" type="hidden" /> <input name="txtHidden" id="txtHidden" type="hidden" /> <input name="__EVENTTARGET" id="__EVENTTARGET" value="" type="hidden" /> <input name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" type="hidden" /> <input name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTk2ODkxNjUxMg9kFgICAw9kFgwCAQ8PZBYCHgpvbmtleXByZXNzBRZyZXR1cm4gb25lbnQoZXZlbnQsMSk7ZAIDDw9kFgIfAAUWcmV0dXJuIG9uZW50KGV2ZW50LDEpO2QCCw8PZBYCHgdvbmNsaWNrBRJyZXR1cm4gdmFsaWRhdGUoKTtkAg8PDxYCHgRUZXh0BS ...